Frequency modulation receiver



Aug. 7, 1945.

J. M. JADRAQUE ETAl.

FREQUENCY MODULATION RECEIVER Filed May 26, 1944 FREQUENCY COU NTER DDSCRHVIINATOR 46 SQUELCH clRculT lll/IS JOSE CAVALLEZBO INVENTORS.

ATTORNEY Patented Aug. 7, 1945 FREQUENCY MODULATION RECEIVER Joaquin Maria J adraque and Luis Jose Cavallero, Buenos Aires, Argentina, assignors to Hartford Y National Bank and Trust Company, Hartford,

Conn., as trustee Application May 26, 1944, Serial No. 537,448

(Cl. Z50-20) 2 Claims.

The present invention relates to frequency modulation receivers and more particularly to frequency modulation receivers which includ-e a squelch or muting system.

As is well known, the main purpose of squelchl or muting systems is to suppress the output of the receiver when the incoming signal decreases to a predetermined value or fades out completely. Generally, the squelch systems are arranged to bias one or all amplifier tubes of the audio-channel to cut-off, when the'intensity of the incoming signal drops below a predetermined threshold value, so that the receiver will remain in a standby condition with the audio-amplifier stage drawing no current from the supply source until the incoming signal increases to a value larger than the threshold of the squelch system.

However, in frequency modulation receivers including frequency counter discriminators of the type described in prior U. S. patent applications of Marc Ziegler etal., Serial No. 464,380, iiled November 3, 1942, and of Marc Ziegler, Serial No. 477,990, filed March 4, 1943, the plate current of the counter tube is relatively large and, in fact, constitutes the major portion of the plate current drawn by the radioand intermediate-frequency amplifier tubes of the receiver. Since in the known receivers, neither the counter tube nor the radioor intermediate-frequency amplifiers are controlled by the squelch system, the main advantage of the receivers of the type referred to consists in the suppression ofthe disagreeable noise heard in the receiver output when the receiver is tuned from one station to another or when no carrier is present. The current consumption of the receiver during the period of standby is nearly the same as during reception, which is particularly disadvantageous in mobile receivers.

We have now found that the above drawback can be eliminated by inserting in the plate supply of one of the audio-amplifier tubes, a relay having one pair of contacts in series with the plate supply of the counter tube and another pair of contacts controlling the platevoltage of the radioand intermediate-frequency amplifier stages, so that the counter tube is cut off and the plate voltage of the radioand intermediate ampliiier stages is considerably decreased when the squelch system blocks the audio-amplifier stage. Consequently, the current drain of the receiver is considerably lower during standby than during reception and in mobile installations batteries of smaller capacity can therefore be used.

An object of the present invention is to provide a frequency modulation receiver having a low current consumption during the standby period when no signal is present or when the incoming signal drops below a predetermined value.

Other objects and advantages will hereinafter appeal.

'Ihe drawing is a simpliiied, diagrammatic showing of a frequency modulation receiver constructed and operating in accordance with our invention.

In the drawing, the reference numeral I0 designates a radio-frequency ampliiier provided with an antenna II and connected to a iirst mixer stage I2. A first heterodyne oscillator I3 provided with frequency controlling means in the form of a reactance tube I4 also is connected to the iirst mixer stage I2, the output circuit of which is coupled to an intermediate frequency amplifier I5. The first intermediate frequency amplifier is coupled to a second mixer stage I6 coupled to a second heterodyne oscillator I1, and the second intermediate frequency, generated in the output of second mixer Iii` is amplified in a second intermediate amplifier stage I 8 and is applied to the input'of a frequency counter discriminator designated with the general reference numeral I9.

Discriminator I 9 shown in the drawing is constituted by a compensated frequency counter similar to that fully described in the above-noted application of Marc Ziegler, Serial No. 477,990. The discriminator comprises a thermionic counter tube 20, the control grid 2I of which is connected to the output of second intermediate frequency amplifier stage I8, While the anode 22 of the tube is connected through a current limiting resistor 23 to one endv of primary winding 24 of a transformer T. The other end of primary winding 24 is connected by means of conductor 25 to the make-contactl 26 of a relay R. The winding of relay R is inserted in series with the primary winding 21 of audio-output transformer T' connected to the anode 28 of the amplifier tube 29 in the audio-amplifier stage 30, coupled by means of said transformer T to a loudspeaker L.

`Movable contact 3| of relay Ris connected through conductor 32 to the positive pole 33 of a plate supply battery not shown in the drawing. Relay R is further provided with a movable contact 34 and xed make and break contacts 35 and 36, respectively, `the former being directly connected to conductor 32 and hence to the positive pole 33 of the supply battery, while the latter is connected to positive pole 33 through a voltage dropping resistor 36. Movable contact 34 is connected by means of conductor 3l to the plate supply connections 3l of the radio-amplifier stage lll, intermediate-frequency ampliiier stages I2 and I8, and mixer stages I2 and I6. Hence, relay R controls the plate tension of the radioand intermediate-frequency stages of the receiver in accordance with the position of movable contact 34.

The secondary winding 38 of transformer T is connected in a rectier circuit arrangement including a diode rectifier tube 39 and load resistor 40. The junction point between diode cathode and load resistor 40 is connected to the junction point of two resistors 4I and 42, the former being connected to plate supply conductor y25, while the latter is connected to ground. The resistors thus constitute a potentiometer from which the compensating tension for the counter rectifier circuit is derived.

The junction point of load resistor 40 -and'secondary winding 38 of transformer T is connected to a ylter 43 having two outputs, one being coupled to the control grid 29 of audio-amplifier tube y29, lwhile the other isc'onne'cted to reactance tube I4. so that the mean frequencyof adjustable oscillator :I3 'is maintained on a 'predetermined frequency difference with' respect to the mean frequency of the incoming signal.

The output circuit of intermediate frequency amplier l`8 is also coupled yby means of conduct-or 44 and coupling capacitor 45 to a squelch system vdesignated with 'the general reference numeral lf3 and 'including a diode 41 closed on a load resistor A48. The anode '49 of diode 41 is connected to said 'coupling capacitor 45 and to filter circuit formed of a series resistor and by-pass 'capacitor 5l, so that a negative direct voltage proportional to the amplitude of the incoming signal is applied to control grid 52 of a sque'lc'h triode53, the anode 54 fof which is connected through vconductor I55 and grid leak resistor 5S 'to control grid 2'9 of audio-amplier tube 29. Anode 54 of squelch triode l53 is further connected lto ground 'and to the positive `pole 33 of 'the plate supply through resistor 56' and plate resistor 5l, respectively. Control grid 52 of squelch triode 53 is connected to the movable arm of a potentiometer P inserted between positive pole 33 and ground and controlling 'the threshold oi the squelc'l'l system, since for all `signals generating 'a negative direct voltage lower than the positive bias applied to the grid 52 through potentiometer P, the plate current of triode 53 flowing through plate resistor 5'! will generate 'a relatively high negative bias voltage at control grid 29 of audio-amplifier tube 29 so that no signal will be vreproduced by loud-speaker L. At the same time, relay R is rendered inoperative due to the considerable decrease in `plate current of audio-amplifier ktube 29, and movable arm 3| disconnects the plate supply 'for countertube 20. Simultaneously, a'lower plate tension is applied to ythe radio mixer and intermediate-frequency 'amplifier stages of the 'receiven'sincethese stages are connected to the positive pole 33 of the plate supply through voltage dropping resistance 36.

Therefore, during standby periods, the current drain of the receiver is very low, and may constitute only a third of the current drawn during reception. Nevertheless, the sensitivity of the receiver is still suilicient to operate squelch system 45 as soon as the intensity of the incoming signal increases above the threshold, since frequency modulation receivers are designed to saturate the limiter of Afrequency counterstage with signals as low as 0.1 'microvolt As soon as the intensity of the incoming signal increases to a value above the threshold of the squelch system, squelch triode 53 is biased to cutoff by the negative voltage at its control grid 52. Consequently, the high negative bias is removed from control grid 29 of audio-amplier tube 29, the plate current of which operates relay R, so lthat movable contact 3l completes the plate supply circuit of countertube 20, while movable contact 34 is 'moved to make-contact 35, thus connecting the full plate supply voltage to the radio frequency mixer and intermediate frequency stages of the receiver.

While we have indicated and described a system for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organization shown and described, but that many modiflcationsmay be made without departing from the scope of the invention, as `set Vforth inthe appended claims.

We claim: v

1. In a receiver for frequency modulated sigvnals comprising radio-'frequency amplifying and heterodyning means connected to a frequency discriminator including 'a thermionic countertube, audio-amplifying and reproducing means connected 'after 'said frequency discriminator, means to produce a 'direct voltage substantially proportional to vthe amplitude of the incoming signal, means responsive yto said direct Voltage to render `inoperative said audio-amplifying 'means at amplitude values of `the incoming signal 'smaller than a `predetermined value, and means operatively connected to Asaid audio-amplifying means to render inoperative said frequency'counter-tube and to lsimultaneously lower vthe plate lvoltage of said radio-frequency amplifying and heterodyning meansat amplitude values of said incoming signal smaller than said lpredetermined value. y

2. In a receiver for frequency 'modulated signals comprising rradio-frequency amplifying Vand heterodyning means connected to a frequency discriminator including a thermionic counter tube, audio-frequency amplifying and reproducing Vmeans connected aftersaid discriminator and constituted ofa thermionic audio-amplifier tube coupled to fa loudspeaker, `means connected ,af-ter said heterodyning means to produce -a direct voltage substantially proportional to the amplitude of -the incoming signal, means responsive to said direct voltage to render inoperative said audioamplifier tube at amplitude values of the incoming ysignal smaller Athan a .predetermined value, an electromagnetic relay having a winding inserted in the plate supply circuit of said audioamplifier tube, a set oflmake-contacts inserted in the plate supply circuit of said thermionic counter-tube, to vrender inoperative-said countertube,.-at amplitude values of the incoming signal smaller than said predetermined value, and a set of make and break contacts having a movable contact arm connected -to 'theplate circuit of said radio-frequency amplifying and heterodyning means, the vmake contact thereof Abeing directly connectedto the positive pole kof the plate su'pply-source and thebreak contact 'being connected to said positive pole through 4a voltage dropping resistor to simultaneously lower the plate voltage of said radio-frequency amplifying and heterodyning means when the amplitude'of said 'incoming signal is smaller than said predetermined value.

. JOAQUIN MARIA JADRAQUE.

LUIS JOSE CAVALLERO. 

